$design-width: 750;
@function px2rem($px) {
  // 计算出是几个rem
  @return calc($px / ($design-width / 10)) + rem;
}

.renderInput {
  color: red;
  background-color: yellow;
  // 会将他的转化为5rem
  width: px2rem(375);
}

.renderInput2 {
  font-size: 20px;
  width: 375px;
  background-color: red;
}

h1 {
  font-size: px2rem(12);
}